1. Field of the Invention
This invention relates to an electron-beam exposure system, a mask for electron-beam exposure and a method for electron-beam exposure, which are employed for manufacturing a semiconductor device. In particular, it relates to an electron-beam exposure system, a mask for electron-beam exposure and a method for electron-beam exposure, which are suitable for proximity effect correction.
2. Description of the Related Art
In electron-beam exposure, proximity effect due to scattered electrons in a resist layer and a substrate considerably affects a pattern linewidth accuracy. Proximity effect correction is, therefore, one of the most important technical elements.
In cell projection lithography which is the most popular electron-beam exposure process, a dose compensation method has been employed, which requires a complicated calculation by the self-consistent method using the exposure intensity distribution (EID) function or the pattern density method.
On the other hand, in scattering with angular limitation in projection electron-beam lithography which has attracted attention as a next-generation electron-beam exposure technique, proximity effect has been corrected by a compensation method using a part of scattered electrons as a correction beam to which GHOST technique is applied (i.e., SCALPEL(copyright) GHOST technique).
A scattering-angle limiting type of electron-beam exposure process employs a segmented transfer technique where a given pattern of a whole chip to be formed or one of its several regions are divided into a plurality of segments; a mask is made with a partial pattern for each segment; and using the masks, exposure is conducted for individual segments to transfer the partial patterns and finally to transfer the given pattern on a wafer.
A mask used for the scattering-angle limiting type of electron-beam exposure process is one in which a pattern consisting of an electron-beam scatterer is formed on an electron-beam transmittable membrane which does not significantly scatter electrons (hereinafter, referred to as a xe2x80x9cscattering membrane maskxe2x80x9d). A wafer is exposed to an electron beam which is not scattered or scattered with a relatively small angle after passing through the electron-beam transmittable membrane. Thus, the difference of electron-beam scattering between the membrane and the scattering regions permits a figural contrast to be formed on the wafer.
The proximity effect in the scattering-angle limiting type of electron-beam exposure process is corrected by selectively transmitting a part of electrons significantly scattered by the scatterer on the scattering membrane mask into an annular opening formed in a limiting aperture disposed at a crossover plane; defocusing the transmitted scattered electrons approximately to a back-scattering range by spherical and chromatic aberration of an object lens; and then irradiating the wafer with the electrons as a correction beam. Such a proximity effect correction technique has been reported in G. P. Watson et al., J. Vac. Sci. Technol., B, 13(6), 2504-2507 (1995). This method is characterized in that a proximity effect is corrected by performing correction exposure simultaneously with the pattern exposure, whereas in a conventional GHOST technique, a wafer is separately exposed to a defocused beam of a reverse pattern for an original exposure pattern. Thus, such proximity effect correction by simultaneous correction exposure with the pattern exposure may considerably contribute to improvement in a throughput.
The conventional proximity effect correction technique in a scattering-angle limiting type of electron-beam exposure process, however, has the following problem.
The extent of a proximity effect depends on a substrate type and a mask pattern. Therefore, when performing exposure using a substrate consisting of a different material or a mask having a different pattern, a correction dose must be readjusted for proximity effect correction suitable for the substrate or the mask. When using a mask having a different electron-scatterer thickness, a limiting aperture must be replaced to one with a different opening size. The correction dose is, however, adjusted by changing dimensions such as the size and the width of the annular opening formed in the limiting aperture. To optimize the correction dose, it is, therefore, necessary to prepare another limiting aperture, which must be then placed after stopping electron-beam exposure and breaking vacuum by opening the chamber in the air. Thus, there has been the problem that according to the conventional process, attempting to optimal proximity effect correction involves a significantly reduced throughput.
Furthermore, the above scattering membrane mask used in the conventional scattering-angle limiting type of electron-beam exposure process has the following problems.
First, since transmitted electrons are also scattered in an electron-beam transmittable membrane, the energy distribution of the image-forming electrons spreads, which causes chromatic aberration, leading to beam blur. For minimizing the beam blur, a beam convergent semi-angle must be reduced. Reduction in a beam convergent semi-angle, however, makes Coulomb effect significant, resulting in a reduced resolution. The Coulomb effect may be minimized by reducing a beam current. It, however, leads to a longer exposure and therefore a reduced throughput. Thus, a scattering membrane mask has not provide adequate electron exposure properties.
Second, the scattering membrane mask is prepared by forming, on a thin (about 100 nm) silicon nitride film, a thinner (about 50 nm) patterned heavy-metal film such as tungsten. Thus, its preparation is very difficult and is of a poor yield.
Besides the above problems, the above proximity effect correction technique has the following problem.
When an underlying pattern such as an interconnection consisting of a heavy metal such as tungsten is formed on a base layer in a resist layer on a wafer surface, image-forming electrons are reflected or back-scattered by the underlying pattern. As a result, there generates a difference of an extent in proximity effect between the resist region over the region without the underlying pattern and that over the region with the pattern. In the conventional proximity effect correction method, it has been difficult to adjust a correction dose for each region in response to the underlying pattern, and furthermore, no such attempts have been conducted.
A mask used in a conventional cell projection or a system used therein (a cell projection type of electron-beam exposure system) will be described.
A conventional cell projection (or exposure system) generally uses a mask which is prepared by forming an opening pattern on a substrate which blocks an electron beam, such as a silicon substrate having a thickness of at least 20 xcexcm (hereinafter, referred to as a xe2x80x9cstencil maskxe2x80x9d).
As a pattern has become finer in response to a more integrated semiconductor device, a stencil mask consisting of a thick substrate has become suffering from the following problem. In preparing a mask, it is difficult to accurately form an opening pattern on a silicon substrate as thick as at least 20 xcexcm, leading to dimensional variation. Furthermore, in electron-beam exposure, the mask absorbs the electron beam and is thus heated, leading to its reduced durability, and is thermally expanded adequately to vary the mask position. In addition, it is required to increase an acceleration voltage for improving a resolution by reducing an aberration in the electron optical system, and therefore, the mask substrate has become thicker, causing these problems more significant.
A thinner mask may improve linewidth accuracy in the opening pattern and reduce heating while electrons to be blocked pass through the mask substrate region (non-opening region). As a result, a region in a wafer not to be exposed is exposed, leading to poor contrast and a reduced resolution.
For solving the problems, JP-A 10-97055 has disclosed a mask for electron-beam exposure prepared by a process comprising the steps of forming an opening pattern on a relatively thin mask substrate and forming an electron-beam scattering layer on the rear surface of the mask for scattering the electron beam passing through the mask. The electron-beam scattering layer may be a polycrystal layer such as polycrystal silicon, tungsten silicide, molybdenum silicide and titanium silicide, or a corrugated shape of layer. The specification describes that such an electron-beam scattering layer may be formed to scatter electrons passing through the pattern layer in the mask (non-opening substrate region) and to prevent them from reaching the wafer.
JP-A 6-163371 has disclosed an electron-beam drawing apparatus characterized in that an opening is formed on a substrate having a thickness shorter than an electron penetration depth to provide an electron-beam shaping aperture used as a mask and a mechanism for blocking scattered electrons passing through the substrate area of the shaping aperture (mask) is provided. In the disclosed invention, a mechanism for blocking scattered electrons passing through the substrate region of the shaping aperture is provided, i.e., a limiting aperture plate with a small opening size is provided in a crossover plane to transmit only an electron-beam passing through the mask opening and to remove electrons scattered in the mask substrate region. In addition, another blocking mechanism has been disclosed, in which an energy filter is formed to change the direction of decelerated electrons losing a part of their energy after passing through the mask substrate region, which is then removed by the limiting aperture.
JP-A 6-163371 has described that the above shaping aperture (mask) can solve the first problem in a mask used in an electron-beam projection lithography apparatus where a large shape transfer mask is exposed to a uniform electron beam to perform exposure for a large shape at one time, i.e., a mask prepared by forming a pattern consisting of a heavy metal which can considerably scatter an electron beam, on a relatively transparent film to an electron beam as a supporting layer.
An objective of this invention is to provide a scattering-angle limiting type of electron-beam exposure process and a scattering-angle limiting type of electron-beam exposure system where proximity effect correction can be adjusted without significant reduction in a throughput and with excellent linewidth accuracy.
Another objective of this invention is to provide a stencil-type mask whereby pattern exposure suitable for a scattering-angle limiting type of electron-beam exposure can be performed simultaneously with proximity effect correction. A further objective of this invention is to provide a mask which can be readily prepared with an accurate mask pattern and which allows pattern exposure to be performed with improved resolution and accuracy. A further objective of this invention is to provide a mask allowing proximity effect to be optimally corrected in accordance with an underlying pattern in a wafer.
Another objective of this invention is to provide an electron-beam exposure system and an electron-beam exposure process exhibiting excellent exposure performance with good resolution and pattern accuracy and allowing proximity effect to be corrected simultaneously with the pattern exposure, leading to an improved throughput. A further objective of this invention is to provide an electron-beam exposure system and an electron-beam exposure process allowing proximity effect to be optimally corrected in accordance with an underlying pattern in a wafer.
This invention provides a scattering-angle limiting type of electron-beam exposure system having a mask comprising a scattering region and a limiting aperture which controls the amount of scattered electrons passing through the mask, comprising
a first limiting aperture fixed at or near a crossover plane and having a central opening and a closed elongated opening surrounding the central opening; and
a second limiting aperture shiftable along an optical axis and having a central opening and a closed elongated opening surrounding the central opening.
This invention also provides a scattering-angle limiting type of electron-beam exposure system having a mask comprising a scattering region and a limiting aperture which limits the amount of scattered electrons passing through the mask, where the limiting aperture comprises a central opening and a closed elongated opening surrounding the central opening, and comprising
a limiting-aperture changing member in which are disposed or fabricated a plurality of limiting apertures different in the size of the closed elongated opening; and
a mechanism which operates the limiting-aperture changing member to dispose a desired aperture among the plurality of limiting apertures in the optical system.
This invention also provides a scattering-angle limiting type of electron-beam exposure system having a mask comprising a scattering region and a limiting aperture which limits the amount of scattered electrons passing through the mask, comprising
a first limiting aperture having an opening only in its center and fixed in the optical system;
a limiting-aperture changing member in which are disposed or fabricated a plurality of second limiting apertures having an opening in their center with a size different from each other and larger than the outer diameter of the first limiting aperture; and
a mechanism which operates the limiting-aperture changing member to dispose a desired second limiting aperture among the plurality of second limiting apertures on the axis of the first limiting aperture.
This invention also provides a scattering-angle limiting type of electron-beam exposure system having a mask comprising a scattering region and a limiting aperture which limits the amount of scattered electrons passing through the mask, comprising
a first limiting-aperture changing member comprising a plurality of first limiting apertures having an opening only in their center, with a outer diameter different from each other;
a second limiting-aperture changing member in which are disposed or fabricated a plurality of second limiting apertures having an opening in their center with a size different from each other and larger than the outer diameters of the first limiting apertures; and
a mechanism which operates the first and the second limiting-aperture changing members to dispose a desired first limiting aperture among the plurality of first limiting apertures and a desired second limiting aperture among the plurality of second limiting apertures on the same axis in the optical system.
This invention also provides a scattering-angle limiting type of electron-beam exposure process using the scattering-angle limiting type of electron-beam exposure system of this invention, comprising the step of
shifting the second limiting aperture along the optical axis as long as the limiting aperture does not block the center of the electron-beam trajectory forming an image corresponding to the outermost periphery of the pattern on the mask, to adjust the amount of scattered electrons passing through the openings of the first and the second limiting apertures for controlling a correction dose and correcting proximity effect simultaneously with the pattern exposure.
According to this invention as described above, proximity effect correction can be readily adjusted and an improved throughput and excellent linewidth accuracy can be achieved, especially in a patterning step in manufacturing a semiconductor device.
This invention also provides an electron-beam exposure mask used in a scattering-angle limiting type of electron-beam exposure process comprising the step of using a mask having a scattering region to perform pattern exposure by means of a scattering contrast induced by difference in electron-beam scattering while utilizing a part of the scattered electrons by the mask to correct proximity effect, where
a mask substrate has a scattering region with a thickness shorter than an electron penetration depth, which includes a region corresponding to a back-scattering range of the image-forming electrons in the wafer; and
a patterned opening is formed in the scattering region.
This invention also provides the above electron-beam exposure mask where an electron scattering layer is formed at least in the scattering region in the mask substrate.
This invention also provides the above electron-beam exposure mask where the thickness of the scattering region varies depending on back-scattering to which the underlying pattern of the wafer contributes.
This invention also provides a scattering-angle limiting type of electron-beam exposure system comprising the above electron-beam exposure mask and a limiting aperture having a central opening and a closed elongated opening surrounding the central opening for limiting the amount of transmitted mask-scattered electrons, which can utilize a part of the scattered electrons to correct proximity effect simultaneously with the pattern exposure.
This invention also provides a scattering-angle limiting type of electron-beam exposure process comprising the step of using a mask having a scattering region to perform pattern exposure by means of a scattering contrast induced by difference in electron-beam scattering while utilizing a part of the scattered electrons by the mask to correct proximity effect, where the mask is prepared by forming a scattering region on a mask substrate, with a thickness shorter than an electron penetration depth, which includes a region corresponding to a back-scattering range of the image-forming electrons in the wafer; and forming a patterned opening in the scattering region.
This invention also provides the above electron-beam exposure process where the thickness of the scattering region varies depending on back-scattering to which the underlying pattern of the wafer contributes.
This invention as described above can provide a stencil-type mask suitable for a scattering-angle limiting type of electron-beam exposure process and allowing proximity effect to be corrected simultaneously with the pattern exposure. The mask of this invention can be readily prepared with an accurate mask pattern and allows pattern exposure to be of high resolution and high accuracy. Furthermore, this invention can provide a mask whereby proximity effect can be optimally corrected in accordance with an underlying pattern in a wafer.
In addition, this invention can provide an electron-beam exposure system and an electron-beam exposure process with an improved throughput and with higher resolution and pattern accuracy, allowing proximity effect to be corrected simultaneously with the pattern exposure. Furthermore, this invention can provide an electron-beam exposure system and an electron-beam exposure process, whereby proximity effect can be optimally corrected in accordance with an underlying pattern in a wafer.